Conventional ultrasound beamforming techniques assume that received acoustic reflections are from diffuse reflectors that reflect ultrasound energy in substantially all direction. This assumption proves useful and effective when imaging soft tissue in a patient. However, the underlying physics for specular reflections is significantly different than for diffuse reflections. A specular reflection is a mirror-like reflection obtained from insonifying a hard level surface with ultrasonic energy. Specular reflections are common when imaging metal objects, including interventional devices and implantable devices. Instead of reflecting ultrasound energy in substantially all directions as is the case with a diffuse reflection, specular reflections are typically very strong at positions where an angle of reflection of the reflected beam is equal to an angle of incidence, and specular reflections generate very little signal at most other locations.
Specular reflectors may contribute to imaging artifacts including a haze artifact in the region close to the specular reflector. If the specular reflector is thin enough, it may also contribute to a ringing artifact that is produced from ultrasound waves that are reflected back-and-forth within the specular reflector. Both the haze artifact and the ringing artifact may degrade any resulting ultrasound images and, in extreme cases, they may even lead to clinicians making inaccurate conclusions based on the ultrasound data.
It is desirable to use ultrasound imaging to track the real-time position of interventional devices such as catheters, guide wires, needles and other devices, which are typically specular reflectors. Conventional ultrasound imaging systems may receive very strong reflected signals from specular reflectors when the specular reflector is perpendicular to a transducer array of the system. In situations where the specular reflector is positioned so that very little or none of the reflected ultrasound energy hits the transducer array, it will not be possible to image the specular reflector. However, according to yet other situations, some of the specularly reflected ultrasound energy may hit the array. This will result in a very intense signal in just a few of the channels corresponding to elements where the angle of incidence equals the angle of reflection. However, in all other channels there will be very little ultrasound signal received from the specular reflector. Standard beamforming techniques assume that that the reflectors behave as diffuse reflectors. As such, standard beamforming techniques typically sum signals from a plurality of channels in order to form an ultrasound image. While this approach has proven very effective for soft tissue and other circumstances where the imaged material behaves like a diffuse reflector, it is ineffective when imaging specular reflectors. The specular reflector will not contribute significant signal to elements other than the elements where the angle of incidence is equal to the angle of reflection. If a conventional beamforming technique is applied to ultrasound data including a specular reflection, the contributions of the specular reflector tend to get minimized during the summing process. Therefore, conventional beamforming techniques are not effective for imaging specular reflectors.
It is often desirable to display the position of an interventional device on an ultrasound imaging system. Conventional systems may use an external tracking system, such as an electromagnetic tracking system or an optical tracking system to determine the location of an interventional device in real-time. However, using an external tracking system adds additional expense and complexity to the entire system. Additionally, the ultrasound system is required to be configured to interface with the tracking system if data showing the location and/or the trajectory of the interventional device is to be displayed in real-time.
It is also known to use a needle guide that acts as a fixture keeping the probe in a constant relative position with respect to a needle being imaged. While this technique is effective for imaging needles, the needle guide combined with the probe and the needle is bulkier and potentially more difficult to maneuver than a stand-alone needle. Additionally, this technique does not work to track other types of interventional devices that are disposed completely within the patient.
For these and other reasons an improved method and ultrasound imaging system for tracking specular reflectors and performing an action based on the position and/or the orientation of the specular reflector is desired.